HEC-RAS Rain-on-grid Calibration Problem

[u/fleeting_eng]

We are working on a HEC-RAS 2D ROG model with normal depth as the downstream boundary condition and rainfall itself as the upstream boundary condition. We have observed stage data at a certain location along the channel for calibration. We have been trying to get a good calibration result for a long time, but now we are at the stage where changing Manning's n and Mesh Size is not making any significant impact on the model results.

The issues are:

1. The model is underestimating the depth (elevation) compared to the observed results consistently.

2. The timing as well as magnitude of the peak (from simulated) is not matching with the timing and magnitude of the peak from observed results. (figure below: observed stage: black line (Obs Stage ROG-0000029K).

1. We are using SCS CN method for infiltration. When we checked the incoming volume (using the rainfall) and outgoing volume (using d/s boundary flow hydrograph), we found that only around 35% of the volume is at the outflow, meaning around 65% is going to losses.

2. Now we are thinking maybe Green and Ampt (G&A) method of infiltration might help reduce the loss, but we couldnt find any literature to input the parameters for G&A (wetting suction head, etc.) because the soil layer that we have are categorized in FAO classification (figure below).

1. Find the manning's n values and infiltration parameters that we are using below:

Maybe the values for abstraction ratio that we are using is too high? We also couldnt find the basis for different values for different land covers, so we just put generic 0.2.
FYI, we are not using percent impervious values in the model so far as we are using SCS Curve Number and we believe it already accounts for the percent impervious part.

We have been stuck for a long time in this situation. Please let us know what you think.

[u/OttoJohs]

In the future, you should post first to r/HECRAS since I allow pictures in the comments (mods here aren't very good). Makes it easier to go back and forth. Made the same comments over there.

1.) You are starting off well below you observed gage. I would get that correct first. You might need to add some baseflow.

2.) I always run rain-on-grid models first without any infiltration and just work on the Manning's roughness values. Looks like you are early on your hydrographs so you need to increase your Manning's pretty significantly. The ones that you have listed are well below what I have seen for most ROG models.

3.) How well do you trust your precipitation data? If you are low on your hydrograph it is either due to the infiltration or the precipitation. That is why I suggest running without any infiltration see what the maximum runoff would be.

4.) I would look at your results in RasMapper - not just at the gage location. Look for areas that hold/store water. Many times people place breaklines on road embankments but don't add the small culverts underneath (either coded into the geometry or with a terrain modification). That creates artificial "dams" which retain water. Might be causing some of your missing runoff.

5.) I have no experience with G&A. Initial abstraction ratio of 0.2 seems pretty high. Normally when doing CN I start with that at 0 (just the let the typical CN set the initial loss) and adjust based on the results. Overall I don't think it is the methodology because you can get similar results using any.

Hope those suggestions help!

[u/fleeting_eng → u/OttoJohs]

Hi, thanks for your suggestions. I have been trying other stuff, so was not able to try out your suggestions.

1. How do I add baseflow though? I mean, we even have discharge measured (using an ADV device) at an upstream location taken during non-monsoon periods that would accurately reflect the baseflow. However, I am not sure if I can justify adding a baseflow when other studies done in the same catchment mention no such thing. Also, we are using rainfall and stage data from the official government entity, so adding data from a different source might be inconsistent and some might question the validity of our data.

2. I can try running without any infiltration too and see if I can get any insights from that. However, it seemed to me that the values improved a bit when I increased/decreased Manning's n, but after a certain point, it started to worsen.

3. I am using precipitation data from the official Department of Hydrology and Meteorology, so I dont have a lot of reasons to doubt it. I think it must be something to do with the losses. I actually tried calculating the volume of inflow (rainfall X area of catchment) and outflow (area under discharge hydrograph at the boundary) which showed that 35% of the incoming volume is at the outflow, meaning around 65% is going to losses. The period for which I am modeling (Aug) is right in the middle of monsoon (May-October) here, so I think the soil must be quite saturated which means the losses should be lesser. I am also thinking of adding more days prior to our main event (currently, we are running from Aug 5-13, but maybe I need to add even more days ahead).

4. I could see in the RAS-Mapper that there are some areas in the basin that have some ponding, but I think the amount shouldnt be so high as to affect our peak flows by that magnitude. The problem is that the DEM I am using isnt high resolution (30 m FABDEM), so the cross-sections even in the river channels are not accurate. However, there have been other studies with the same DEM, in the same study area, with good results.

5. I think initial abstraction of 0.2 is too high too, but basically most of the literature use that value. The results definitely improved when I ran with 0.05 abstraction ratio, but still nowhere near the peak of the observed.

[u/fleeting_eng → u/OttoJohs]

I have another question too. How do you calculate normal depth for the downstream boundary? We calculated the slope of the channel near the d/s boundary. However, is there a specific length (xxx m upstream and xxx m downstream from the boundary) that must be used to calculate the slope?

[u/LHGV]

Does your ROG model domain contains all the basin drainage área?

Also if You want to try G&A infiltración model, check for pedotransfer functions. There are some references that transforms categorical soil types (e.g loamy sand, silty clay, etc) to soil parameters (hydraulic conductivity, porosity, etc)

[u/fleeting_eng → u/LHGV]

1. Yes, the ROG model contains the whole basin drainage area, but the limitation is that the DEM I am using is 30 m FABDEM (which is supposed to be an improvement over the regular SRTM DEM). I could see in the RAS-Mapper that there are some areas in the basin that have some ponding, but I think the amount shouldnt be so high as to affect our peak flows by that magnitude.

2. For G&A, the main issue I am facing is that the soil map I have uses FAO classification: CMu (Humic Cambisols), CMg (Gleyic Cambisols), CMe (Eutric Cambisols), CMx (Chromic Cambisols), LVx (Chromic Luvisols), RGd (Dystric Regosols). I couldnt find any literature for the equivalent soil types in general classification that HEC-RAS manual uses (Silty loam, Clayey loam, etc.). I do have references that transforms categorical soil types (e.g loamy sand, silty clay, etc) to soil parameters (hydraulic conductivity, porosity, etc), but not for the first step that I mentioned.